Embodiments of the present invention relate generally to the field of computerized tomography (CT) and, more specifically, to an object identification method in dual-energy CT scan images.
Scan images of a target object obtained through computerized tomography (CT) technology include a plurality of pixels of different gray scales. The gray scales of the pixels are proportional to the density of the target object to be scanned. Areas of different densities in the target to be scanned differ in terms of the level of X-ray absorption. Therefore, in a CT scan image, pixels having a low gray scale are used to represent low density area having a lower X-ray absorption level, and pixels having a higher gray scale are used to represent high density area having a higher X-ray absorption level. Usually, a CT value (unit: Hu) is used to represent the pixels in a CT scan image. For example, the CT value of pixels corresponding to an object or tissue with a higher level of X-ray absorption is set to be higher than the CT value of pixels corresponding to an object or tissue with a lower level of X-ray absorption. For example, the CT value of pixels corresponding to bone tissue is set as +1000 Hu, the CT value of pixels corresponding to water is set as 0 Hu, the CT value of pixels corresponding to the air is set as −1000 Hu, and so on.
Currently, a dual-energy CT technology has been has proposed. In this technology, X-rays of two different energies are used to scan the target to be scanned, thereby obtaining a dual-energy CT scan image of the target. Such a dual-energy CT scan image contains more information than the scan image obtained by using the traditional single-energy CT technology employing single energy X-rays to scan the target.
However, in both single-energy CT scan images and dual-energy CT scan images, different objects in the target may have the same or similar levels of X-ray absorption. For example, the bone tissue and the calcified part such as blood vessels or other tissues developed due to lesions may have the same or similar X-ray absorption level, and hence the same or similar CT value. Therefore, it is difficult to distinguish objects represented by pixels of the same or similar grayscale values in a CT scan image.
In addition, in order to highlight the different objects in the target, a contrast agent is applied (e.g., injected) to the target before performing a CT scan. The CT scan where a contrast agent is applied to the object is also called a CT contrast enhanced scan, and the CT scan images obtained thereof are also called CT contrast enhanced scan images.
However, in the CT contrast enhanced scan images, because the pixels corresponding to the contrast agent have a relatively high CT value, it is difficult to distinguish the pixels corresponding to the contrast agent from pixels corresponding to bone tissue or other tissues that have a relatively high density and relatively high X-ray absorption level. In the prior art, there is a method for identifying pixels corresponding to the contrast agent by comparing the CT scan images (unenhanced images) obtained by a CT scan before applying the contrast agent (unenhanced scan) with the CT contrast enhanced scan images (enhanced scan image). However, such method requires two CT scans of the target, and thus increases the X-ray absorption dose by the target.
Therefore, there is a need for a method for identifying different objects in a CT scan image.